The purpose of the Animal and Microsurgery Core (Core B) is to establish a focused facility that provides investigators in each of the two projects with sufficient breeding of animals with given genotypes and with a standardized cardiac transplantation and tolerance induction procedure for the proposed experiments. Core B will provide a key service role that will enable the Principal Invesfigators and their staff to focus on experimental and scientific efforts. Centralized breeding and screening of the progeny will ensure that animals of required genotypes are available for experiments. Transplantafion performed by a centralized source will standardize the procedures, minimize surgically imposed disparities in the grafts used by the two projects and allow some long-term acceptors generated for Project 1 (studying the induction of tolerance) to be used for Project 2 (studying the maintenance of tolerance) to minimize experimental cost. In addifion, purchases of reagents such as anesthetics and immunosuppressants will be done in bulk for both projects to reduce cost. The specific tasks of Core B will be to: a) Ijreed and genotype animals to be used for both projects; b) transplant heart allografts and perform thymectomies as directecj by the project leaders and their personnel; c) prepare and inject donor splenocytes and administer indicated immunosuppressive therapies; d) monitor the beating of the transplanted hearts, the health of the transplant recipients, and record times of rejection; e) sacrifice transplanted animals as directed by the Principal Investigators and their personnel and collect organs for histology, as well as to distribute to other project personnel for phenotypic and functional analysis of immune cells.